In an electrical and electronic system, a current conversion and a voltage conversion are common and important technologies. In prior arts (including “AC-AC voltage conversion”, “DC-DC conversion”, “AC-DC conversion” and “DC-AC conversion”, in which AC means alternating current and DC means direct current), the current conversion and the voltage conversion need to be performed separately rather than simultaneously, which will be described in detail as follows.
In the AC-AC voltage conversion, a voltage conversion is realized by a transformer, and an energy transmission and the voltage conversion are mostly realized by an electromagnetic coupling between a primary coil and a secondary coil. An AC-AC voltage transformer has disadvantages of large volume, heavy weight and low energy density. Moreover, the AC-AC voltage transformer has a specific requirement on a frequency of an AC electricity. In one aspect, the lower the frequency is, the larger the volume and the lower an efficiency will be, and thus the voltage transformer can not be realized under an low frequency AC current; in another aspect, a large electromagnetic loss may be led at an high frequency, and thus the frequency of the AC electricity has to be constrained in a narrow range.
The DC-DC conversion can not be realized by a conversional technology. Recently, it has been found that a DC-DC voltage transformer may be realized by using a power semiconductor device as a switch, a large inductor or capacitor as an energy storage element, and under a control of a driving circuit. However, such method has disadvantages of complicated apparatus, a passive element of large volume and heavy weight, high cost, serious electromagnetic interference and an associated electromagnetic compatibility problem.
In the AC-DC current and voltage conversion, first an AC-AC voltage conversion and then an AC-DC current conversion are realized. The AC voltage conversion is realized by an independent AC transformer, while the current conversion is mostly realized by a bridge rectifier comprising a plurality of diodes. However, the bridge rectifier can only realize the current conversion.
In the DC-AC current and voltage conversion, first a DC-AC current conversion and then an AC-AC voltage conversion are realized. The voltage conversion is realized by an AC transformer, while the current conversion is mostly realized by using the power semiconductor device as the switch and combining a filter which also can only realize the current conversion.